1. Field of the Invention
This invention relates to a transistor amplifier, and more particularly to a low-noise, small-signal amplifier which can be effective used as a preamplifier for an acoustic or audio apparatus.
2. Description of the Prior Art
A low-noise, small-signal amplifier is known in which two bipolar transistors are connected in series between input and output terminals so that a small signal supplied to the input terminal is suitably amplified prior to delivery at the output terminal. In order to improve the distortion factor of such amplifier and to increase the input impedance and reduce the output impedance, a series-at-input and parallel-at-output type signal feedback circuit is provided. Further, in order to improve the bias stability of the bipolar transistors, a DC feedback circuit is provided. In order to avoid an adverse effect from the impulsive external noise signal, a time constant characteristic is provided at the input side of the first stage bipolar transistor.
The foregoing known amplifier is disadvantageous in that it obviously requires a large number of circuit elements and hence its adjustment is relatively difficult to achieve. Further, even if the bipolar transistors are of a low-noise type, the thermal agitation noise of the resistors which constitute the above described series-at-input and parallel-at-output feedback circuit restricts the noise figure level that is attainable in the event of short circuitry on the input side of the amplifier.